This invention relates to ground fault circuit interrupters of the differential transformer type.
Ground fault circuit interrupters are required to respond to a predetermined ground fault current that flows from the hot line conductor on the load side of the current transformer to a ground point. The value of fault current that trips the device is set by components in a sensing amplifier and trip circuit connected with the secondary sensing winding of the current transformer. Some such amplifiers are sensitive only to a single polarity of ground fault current and are quite effective for their intended purpose. One arrangement including a half wave sensitive sensing amplifier is that disclosed in U.S. Pat. No. 3,852,642, by Engel et al which should be referred to for further information and background.
The requirement for miniaturization of the device has led to the practice of making the sensing amplifier as a hybrid microelectronic unit separately packaged with its own encapsulation with leads for external connection but without access to internal connections in the unit. This means that fabrication of such a unit with chosen components sets the trip level and makes it impossible to alter the trip level without very time-consuming operations on the encapsulated package and great risk of the amplifier's destruction.
Ground fault interrupters for personnel protection in the United States have most frequently in the past been made to have a trip level of 5 milliamperes or less and are required not to trip at fault currents less than 2 milliamperes as required by standards of Underwriters Laboratories and other industry standards. Devices for such trip levels are referred to as Class A devices. Recently, however, there has been interest in making such devices with trip levels in the range of from 4 milliamperes to 6 milliamperes which are referred to as "New Class A" devices. It is apparent that a former Class A device does not meet the requirements of a "New Class A" device and packaged sensing amplifiers for former Class A devices have not heretofore been susceptible of any change or modification to permit them to perform within the new requirements. This of course limits the manufacturer's flexibility in that separate and distinct sensing amplifier units are required for each class or trip level for which he intends to make the devices.